The present invention relates generally to a method of manufacturing a laminated rotor for a motor. More specifically, the present invention is related to methods of manufacturing a laminated rotor with laminations having a desired rotor bridge thickness prior to the assembly of the laminated rotor core.
A squirrel cage rotor for use in an induction motor has a rotor core and a rotor cage that extends through the rotor core and is connected together at each end of the rotor core by end rings. The rotor core is typically made of a magnetic material such as iron or steel and the rotor cage is typically made of an electrically conductive material such as copper, aluminum or an aluminum alloy. The rotor core has a substantially cylindrical shape with a longitudinally extending central bore to receive the shaft of the motor and a plurality of longitudinally extending rotor slots or apertures, which rotor slots may be slightly skewed, to receive corresponding rotor bars of the rotor cage. A laminated rotor core is commonly manufactured or formed by stacking or assembling a plurality of discs or laminations of the magnetic material on top of each other until the desired substantially cylindrical shape is obtained. During the stacking or assembling process, the laminations are also aligned or oriented into their proper position. Alternatively, the rotor core can be manufactured from a single piece of the magnetic material, but this technique is less common.
Each lamination in the rotor core is formed or extruded to a pre-selected thickness, shape and configuration. The pre-selected configuration of the laminations includes an aperture for the central bore, a plurality of apertures for the rotor slots positioned equidistantly about the central bore and a predetermined bridge thickness, which bridge thickness is defined as the radial distance between the outer circumference of the lamination and the aperture for the rotor slot. The dimensioning of the bridge thickness is important because the bridge thickness of the rotor is related to the motor's performance, wherein a thinner bridge thickness provides better performance. The pre-selected configuration of the lamination can also include other features as needed. As the laminations are stacked to form the rotor core, they are aligned and/or oriented into an appropriate position to form substantially continuous apertures in the rotor core and, if necessary, other desired features of the rotor core.
Next, the rotor cage is manufactured or formed by positioning or disposing a rotor bar into each of the plurality of rotor slots in the rotor core, which rotor bars extend to at least the ends of the rotor slots, and connecting the adjacent ends of the rotor bars to each other with an end ring. In one technique, the stacked laminations forming the rotor core can be welded together and/or axially compressed to fix their position and can then be placed in a mold. Once in the mold, the rotor bars, and possibly the rings, can then be formed by die casting or injection molding molten aluminum (or other suitable material), under high pressure, directly into the rotor slots and possibly into molds for the end rings. Alternatively, the rotor bars can be placed or positioned in the rotor slots using any suitable technique and can then be connected together by attaching or connecting a ring to each end of the rotor bars using any suitable technique such as brazing. It should be noted that if the end rings are not cast during the casting process, the end rings can be connected or attached using the brazing technique described above.
One potential problem with casting the rotor bars into the laminated rotor core is that additional steps have to be taken to prevent the molten casting material, e.g. molten aluminum, from leaking or seeping between the laminations. To prevent the molten casting material from leaking or seeping between the laminations, the laminations are typically formed or extruded with a greater than desired outer diameter or bridge thickness and are welded together or compressed axially as discussed above. When these additional steps are performed, both the inner diameter and outer diameter of the laminated rotor have to be subsequently machined or processed after the casting process to obtain the desired inner diameter, outer diameter and bridge thickness for the laminated rotor.
Therefore, what is needed are techniques for manufacturing a laminated rotor with laminations having an outer diameter and/or bridge thickness that restricts the molten material cast into the rotor core from leaking or seeping out between the laminations during the casting process.